To add a unit to your Command Center list, click the Options button at the top right. Enter the IP address of that unit and push close. If a connection was made, a unit object should appear on your Command Center page. Otherwise, check to make sure the unit is online and powered on.
There are two ways to remove a unit. You can either click the "X" button found on each unit object on the top right corner, or go to options and enter in the IP address of the particular unit you wish to delete.